Image display apparatuses that express images through the formation of dots on a display medium such as a printing medium or a liquid crystal display are widely used as output devices for various types of image equipment. These image display apparatuses can only express images locally in terms of whether or not a dot is formed, but by appropriately controlling the density of dot formation in accordance with the tone values of the image, images in which the tone changes gradually can be expressed.
In these types of image display apparatuses, the methods by which to determine the dot on-off state for each pixel in order to form dots with the appropriate density in accordance with the image tone values include the method termed the error diffusion method and the method termed the average error minimum method, which is mathematically equivalent to the error diffusion method.
In the error diffusion method, the binarization error occurring due to the formation or non-formation of a dot in a target pixel is diffused among the state-undetermined pixels in the vicinity of the target pixel and stored, and during determination of the dot on-off state for these state-undetermined pixels. the dot on-off state is determined such that the errors diffused from the nearby pixels are eliminated. In the average error minimum method, the binarization error occurring during dot on-off state determination is stored in connection with the target pixel without diffusing it among nearby pixels, and instead, during dot on-off state determination for the state-undetermined pixels, the errors stored in regard to the nearby pixels are read out and the dot on-off state is determined for the target pixel so as to eliminate these errors.
In both of these methods, because the image comprises a large number of pixels, it is impossible to conduct the dot on-off state determination simultaneously for all pixels. At the same time, because the image data is generated through scanning of the original image, the image data is supplied in the order of the pixel rows (‘rasters’) obtained from scanning. For these reasons, the dot on-off state determination is conducted in the order of the rasters included in the image. Namely, the dot on-off state determination is conducted in sequence beginning at one end of each raster, and when the determination for all pixels in the raster is completed, processing for the next adjacent raster is begun. In these methods, because a pixel belonging to a different raster is not processed consecutively even where it is adjacent to the processed pixel in the image, binarization errors arising from dot on-off state determination are stored in an error buffer, and are subsequently read out and used where necessary. Consequently, by conducting the dot on-off state determination so as to eliminate errors while taking into account the binarization errors arising in nearby pixels, dots can be formed at an appropriate density in accordance with the image tone values, thereby enabling a high-quality image to be displayed on the image display apparatus.
However, in these methods, the problem exists that binarization errors arising each time a dot on-off state determination is conducted must be written to the buffer frequently, which means that the dot on-off state determination cannot be carried out quickly. Namely, in the error diffusion method, an error to be diffused among nearby pixels must be written to the error buffer each time dot on-off state determination is conducted, and in the average error minimum method, binarization errors that occur in nearby pixels must be read out from the error buffer each time dot on-off determination is carried out. In either case, dot on-off state determination is time-consuming to the extent that reading and writing to and from the buffer must be carried out frequently. Where dot on-off state determination takes a long time, it becomes difficult to display image rapidly.
This invention was created in order to resolve the problem with the conventional art described above, and an object thereof is to provide a technology that allows a high-quality image to be displayed rapidly by reducing the amount of time required for dot on-off state determination without causing a deterioration in image quality.